1. Field of the Invention
The present invention relates to a rotor angle detecting apparatus for detecting the rotor angle of a DC brushless motor without using a rotor position detecting sensor.
2. Description of the Related Art
In order to obtain a desired torque by energizing a DC brushless motor, a voltage needs to be applied to an armature in an appropriate phase which corresponds to an electrical angle (hereinafter, referred to as a rotor angle) of a rotor having magnetic poles. With a view to reducing costs for a DC brushless motor and a motor drive unit by omitting a position detecting sensor for detecting the rotor angle, there have been proposed many methods for detecting the rotor angle with using no position detecting sensor.
For example, Japanese Patent Unexamined Publication Nos. Hei. 10-323099 and Hei. 11-332279 describe methods for detecting the rotor angle of a DC brushless motor in a motor control apparatus for controlling such a DC brushless motor by a so-called dq coordinate system in which method an estimation alternating current signal voltage is applied in a direction of one of axes to thereby generate on the other axis side a current, by which the rotor angle is detected.
In addition, the inventor et al. of the present invention proposed, in US2002/0149335A1, a rotor angle detecting apparatus for detecting the rotor angle using no position detecting sensor. In the rotor angle detecting apparatus, when an inspection voltage of a high frequency is imposed on a drive voltage which is applied to three phase armatures of a salient pole-type DC brushless motor, a detection value of a current flowing to a first-phase armature and a detection value of a current flowing to a second-phase armature of the three phase armatures and a high-frequency component corresponding to the inspection voltage are used to calculate a sine reference value corresponding to a sine value of an angle which is twice the rotor angle of the motor and a cosine reference value corresponding to a cosine value of an angle which is twice the rotor angle of the motor.
Then, the rotor angle can be detected based on the sine reference value and the cosine reference value with good initial follow-up characteristics and with being affected little by motor parameters.
However, when imposing the inspection voltage on the drive voltage in order to detect the rotor angle, there may occur a case where irksome noise is generated from the motor.